UpdateVirtualSensor uses information from signals

UpdateVirtualSensor uses information obtained from signals to avoid
extensive dbus queries.

Tested:
1:Execute DBus cmd to see if VirtualSensor can

UpdateVirtualSensor uses information from signals

UpdateVirtualSensor uses information obtained from signals to avoid
extensive dbus queries.

Tested:
1:Execute DBus cmd to see if VirtualSensor can correctly obtain the
value of DbusSensor
busctl tree xyz.openbmc_project.VirtualSensor
busctl introspect xyz.openbmc_project.VirtualSensor xxx
2:Waiting for the value change of DbusSensor,Check if the value of
the virtual sensor has changed after the dbusSensor changes.

Fixes openbmc/phosphor-virtual-sensor#1

Change-Id: If11f9017b31ce5cf06f910a38c65637c55d74b24
Signed-off-by: Tao Lin <lintao.lc@ieisystem.com>

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